In both mice and humans, the majority of newly generated B cells are autoreactive and a selection process - tolerance - exists to ensure that most of these cells do not enter the peripheral B cell population. Recent data indicate that in individuals affected by systemic autoimmunity such as lupus, a large fraction of the B cells participating in disease bear germline immunoglobulin genes and are, therefore, the product of a central selection process gone awry. While in the mouse the understanding of how newly generated B cells are centrally selected in the bone marrow is quite sophisticated, there remains a surprising dearth of knowledge about this process in humans. Given the role primary B cells play in autoimmune diseases, it is of critical importance to correct this gap in knowledge. The overall objective of this application is to determine how human immature B cells are selected to enter or not the primary B cell population. The central hypothesis is that a fraction of human autoreactive B cells undergoes tolerance via receptor editing, but that this process is compromised in immune systems prone to the development of systemic autoimmune diseases. This hypothesis has been formulated on the basis of preliminary studies developed with a unique humanized mouse model of human B cell tolerance. The results of these studies have provided us a direct way to investigate directly human B cells undergoing central tolerance to natural self-antigens. We plan to test our hypothesis and attain the objective of this application by pursuing the following specific aims: 1) Determine the phenotype of human B cells undergoing central tolerance to synthetic and natural self-antigens in humanized mice and humans; 2) Determine use and efficiency of receptor editing in human B cells responding to natural self-antigens; and 3) Identify defects in central B cell tolerance in human immune systems genetically predisposed to autoimmunity. The major innovation of this project resides in the ability to investigate human autoreactive B cells directly while they undergo central tolerance. These studies are significant because they provide direct information on mechanisms human B cells utilize during central selection and because they contribute to understanding why and how autoreactive B cells are generated at higher numbers in some individuals predisposed or suffering from autoimmunity. Ultimately, such knowledge has the potential of guiding us toward novel treatments for autoimmune diseases.